Hundreds of millions of people must wear glasses occasionally to correct for any eye impairment. In particular, presbyopia (also called farsightedness or old sight) affects more than 1.2 billion people around the world. Presbyopia normally develops as a person ages, and is associated with a natural progressive loss of accommodation. The presbyopic eye often loses the ability to refocus rapidly and easily on objects at varying distances. There may also be a loss in the ability to focus on objects at near distances. Although the condition progresses over the lifetime of an individual, the effects of presbyopia usually become noticeable after the age of 45 years. By the age of 65 years, the crystalline lens has often lost almost all elastic properties and has only limited ability to change shape. This visual impairment is a permanent source of frustration because of the rapid rise in the use of personal digital display devices in mobile phones, tablet computers, car navigation systems (GPS), and many other devices. It is inconvenient when a mobile phone user suffering from presbyopia receives a call but does not have time to find his glasses to read the display to identify the caller or tries to read from a GPS in an automobile while driving.
Known methods and devices for treating presbyopia seek to provide vision approaching that of an emmetropic eye. In an emmetropic eye, both distant objects and near objects can be seen clearly due to the accommodation properties of the eye. To address the vision problems associated with presbyopia, reading glasses have traditionally been used by individuals to add plus power diopter to the eye, thus allowing the eye to focus on near objects and maintain a clear image. This approach is similar to that of treating hyperopia, or farsightedness. Presbyopia has also been treated with bi-focal eyeglasses, where one portion of a lens of the eyeglasses is corrected for distance vision, and another portion of the lens is corrected for near vision. A more advanced implementation of this principle led to the concept of progressive eyeglasses. Contact lenses and intra-ocular lenses (IOLs) have also been used to treat presbyopia. Other approaches include bilateral correction with either bi-focal or multi-focal lenses. In the case of bi-focal lenses, the lens is made so that both a distant point and a near point can be focused. In the multi-focal case, there exist many focal points between near targets and far targets.
Optically, presbyopia is the result of the visual image being focused behind the retina rather than directly on it causing the image to be blurred. The use of glasses or contact lenses changes the direction of rays of light to cause the visual image to be focused on the retina rather than behind it, resulting in a sharp image. FIG. 1 schematically shows an eye with insufficient refractive power. This is the typical case of a presbyopic person looking at a close object. The light emitted by a point source, instead of being focused on the retina, is focused behind the retina. This results in the person seeing a spot instead of a point. To a first approximation, this spot is a disc. However, effects such as diffraction, monochromatic aberrations like astigmatism, and chromatic aberrations may cause this spot to have a more complex shape. The spot is called the point spread function (PSF) of the global optical system. When looking at a complete image composed of multiple point sources located at the same distance from the eye, each point generates a similar spot at a different position on the retina. These spots overlap, as shown in FIG. 2, causing the impression of blur. Visual impairments other than presbyopia or farsightedness can be described with the same principles.
The mathematical operation corresponding to replacing all points in the image by overlapping spots is called a convolution. Corrective lenses, like reading glasses, add the missing refractive power to the eye so that the image of each point is focused on the retina or close to the retina, yielding a PSF with a small extent. For visually impaired people looking at the screen of an electronic device there has been no effective way to correct for this problem other than optical correction. There has been no effective solution to correct vision directly at the source rather than at the eye level (e.g. glasses).